A communication system has become widely used in which a transmission unit (master) and a plurality of communication terminals (slaves) are connected to a transmission path, and in which respective communication terminals communicate with the transmission unit. As an example of this kind of communication system, it has been known a system in which a transmission unit periodically monitors states of communication terminals, and transmits, upon detecting a change in the state of one of the communication terminals, a signal to other communication terminals so as to respond to the change of the state (see Japanese Patent Publication No. 1180690, Japanese Patent Publication No. 1195362 and Japanese Patent Publication No. 1144477).
However, in the communication system of above described structure, communication speed thereof is relatively low, because the communication terminals always perform communication via the transmission unit and the transmission unit performs polling the communication terminal. Accordingly, this communication system is unsuitable for transmitting information with a relatively large data volume such as an analog quantity. Further, this communication system has a problem that reliability thereof is low, because whole system will halt at the time of a failure of the transmission unit etc.
In view of this problem, there has been proposed such a communication system designed as a mixture of: an existing communication system for performing communication between communication terminals via a transmission unit; and a communication system for performing communication directly between communication terminals in a peer-to-peer mode (P2P) (see Japanese patent application publication No. 2009-225328). In this communication system, a transmission path is shared by: first communication terminals which communicate with each other via the transmission unit (master); and second communication terminals which communicate directly with each other. Therefore, the second communication terminals can be easily added to the existing communication system. In this communication system, the first communication terminals perform communication using a transmission signal (first protocol signal) which is repeatedly transmitted from the transmission unit to the transmission path. The second communication terminals perform communication using a superimposed signal (second protocol signal) which is superimposed on the transmission signal.
Incidentally, the transmission signal is a signal in conformity with a time-division system, and each one frame in the transmission signal is divided into a plurality of time periods in a time axis direction, and a part of the time periods is assigned as superimposable period within which the superimposed signal is allowed to be superimposed (i.e. superimposing period adapted for superimposing the superimposed signal). In this communication system, the second communication terminal performs communication during the part of the time periods of the transmission signal which is assigned as the superimposable period (superimposing period), by use of the superimposed signal which is transmitted through the transmission path shared with the transmission signal.
In this structure, when the transmission data can not be transmitted within a single superimposable period due to large data volume, the second communication terminal divides the transmission data into a plurality of data, and transmits them sequentially for each superimposable period. In this case, the transmission data is transmitted over a plurality of superimposable periods. If the volume of the transmission data becomes large, the second communication terminal may not be able to transmit the transmission data with a single frame of the transmission signal. In this case, the second communication terminal may transmit the transmission data over a plurality of frames.
As described above, the second communication terminal can transmit the superimposed signal only in a part of time period of the transmission signal, and it is therefore restricted the volume of data which can be transmitted within a single frame of the transmission signal. Accordingly, it has been difficult to improve the speed of the communication between the second communication terminals.